1. Field of the Invention
The present invention relates to an optical modulator and an optical signal generation apparatus, and is applicable to, for example, an optical modulator and optical signal generation apparatus that are used in long-distance, large-capacity optical fiber communications or the like and that provide intensity modulation or phase modulation to controlled light in accordance with control light.
2. Description of the Related Art
With the development of network technologies, as represented by the Internet and suchlike, demands for increases in communication capacities of optical fiber communications have been progressively rising.
Increases in communication capacities of optical fiber communications in recent years have been implemented by increases in numbers of wavelength channels capable of communication (for example, WDM: wavelength division multiplexing technologies) and by increases in bit rates of respective wavelength channels.
As technologies that increase bit rates per wavelength channel, multiplexing communication methods such as, for example, TDM (Time Division Multiplexing communications) and the like have been investigated and put into practice. A TDM system is a system that increases the bit rate per wavelength channel by using a time division multiplexed signal in which plural channels are multiplexed in time divisions.
At the receiving side, the TDM system is provided with a multiplex separation section that separates individual channels out from a time division multiplexed signal on the basis of gate signals generated from a clock signal, and implements reception by separately extracting information from the individual channels. A conventionally employed TDM system is a system that performs time division multiplexing signal generation and multiplexed signal separation at the level of electronic devices. Such a system is referred to in particular as electrical TDM.
To increase the bit rate of electrical TDM, increases in speed of electronic devices and opto-electronic devices, such as photodiodes for opto-electronic conversion, semiconductor lasers and the like, are required. Bit rates thereof have a limit at a bit rate of around 40 Gbit/s.
Thus, to further increase the bit rates of TDM systems, it is desirable to implement the aforementioned time division multiplexing signal generation and multiplexed signal separation section with completely optical means. Such a system is referred to in particular as optical TDM.
In an optical TDM system, it is desirable to generate time division multiplexed optical pulse signals using an optical circuit in which, for example, optical couplers and the like are combined. Further, it is desirable to implement separation at the receiving side using all-optical optical switches, gates of which are operated by optical control signals, which are control light. Further yet, at optical relays for long-distance communication, nodes of optical networks and the like, optical signal control technologies are required, for wavelength conversions, the generation of modulated optical signals and the like, and also for optical signal regeneration operations and the like. Similarly, these too are desirably implemented using all-optical wavelength converters, optical modulators and the like, which perform wavelength conversions of controlled optical signals, generation of modulated optical signals, signal regeneration and so forth with controlling optical signals.
That is, in an optical TDM system, in order to implement separation at the receiving side thereof, and optical signal regeneration at optical relays and the like, all-optical optical switches and modulators are required, which perform switching operations and modulated signal generation operations on controlled optical signals with controlling optical signals.
A preferable example of a technology for implementing all-optical optical switches and modulators is a process that utilizes the optical Kerr effect, which occurs in optical fibers.
The optical Kerr effect that occurs in optical fibers is a phenomenon in which the refractive index of an optical fiber is changed by strong-intensity light propagating through the optical fiber. A response rate of the effect is a few femtoseconds (fs). Thus, when the optical Kerr effect in optical fibers is utilized in constituting an optical switch or optical modulator or the like, it is possible to realize optical switches and optical modulators that are capable of switching and modulating optical pulse signals of the order of several hundred Gbit/s and upward.
As an optical switch that utilizes the optical Kerr effect, an optical switch utilizing the optical Kerr effect that occurs in polarization plane-maintaining-type single-mode optical fibers has been investigated, for example, in “Ultrafast Optical multi/demultiplexer utilizing optical Kerr effect in polarization-maintaining single-mode fibers”, T. Morioka, M. Saruwatari and A. Takada, Electronic Letters, Vol. 23, No. 9 pp. 453-454, 1987.
The switch utilizing the optical Kerr effect that is disclosed by Morioka et al. utilizes a polarization-maintaining single-mode optical fiber (hereinafter referred to as a “polarization plane-maintaining optical fiber” or simply an “optical fiber”) as the optical fiber in which the optical Kerr effect occurs.
This polarization plane-maintaining optical fiber has a constitution in which equivalent refractive indices for guided light differ between the direction of an optical axis known as the phase lag axis or slow axis, which is set in a plane orthogonal to the propagation direction of light in the optical fiber (which is hereinafter referred to as an optical axis direction of the optical fiber), and the direction of an optical axis known as the phase lead axis or fast axis, which intersects the slow axis.
Further, an optical fiber utilized in an optical switch that is disclosed by Morioka et al. has a constitution that includes a face at which two polarization-maintaining single-mode optical fibers are fused, with the optical axes intersecting, which enables birefringence characteristics of the polarization plane-maintaining-type single-mode optical fibers to offset one another.
Into the optical switch described by Morioka et al. are input linearly polarized control light, with a polarization plane parallel to an optical axis of a polarization plane-maintaining optical fiber, and linearly polarized signal light (controlled light), with a polarization plane angled at 45° from the optical axis of the polarization plane-maintaining optical fiber.
When an optical pulse constituting signal light and an optical pulse constituting control light are not synchronously input into this optical switch, the signal light optical pulse is output in a linearly polarized state the same as at the input into the light switch. On the other hand, when a control light optical pulse and a signal light optical pulse are synchronously input, the optical Kerr effect is induced by the control light optical pulse for, of polarization components of the signal light optical pulse, a polarization component that is parallel with the polarization direction of the control light optical pulse. That is, a phase shift is caused in the signal light optical pulse by a cross phase modulation effect between the signal light optical pulse and the control light optical pulse, because of the optical Kerr effect.
Herein, a control light optical pulse and a signal light optical pulse being input synchronously signifies a state in which, when an individual optical pulse signal including control light is input into a third polarization plane-maintaining optical fiber 22 which will be described hereinafter, a delay time of the control light pulse signal or the signal light is regulated and the control light pulse signal is input so as to temporally coincide with an individual optical pulse including the signal light. At such a time, as will be described in more detail hereinafter, a walk-off effect can be expected due to group velocity dispersion, so the inputs may be provided with a slight offset between a position of the control light optical pulse and a position of the signal light optical pulse. The meaning of synchronized input states covers states including such cases.
When an amount of the phase shift φ is equal to π, the polarization direction of the signal light optical pulse is rotated through 90° from when it is input into the optical switch. That is, the polarization direction of the signal light optical pulse rotates to a direction at −45° with respect to the optical axis of the optical fiber. Hence, by an analyzer being disposed at the output side of the optical switch, the signal light optical pulse may be transmitted or blocked in accordance with the control light.
That is, if the direction of an optical axis of the analyzer is set to be arranged in an orientation with which the signal light optical pulse is transmitted when the polarization direction thereof has been rotated by 90° from input into the optical switch and the signal light optical pulse is blocked when the polarization direction is the same as at input, only optical pulses whose polarization plane has been rotated by the control light may be transmitted through the light switch, and the signal light optical pulses can be switched.
In realizing the optical switch disclosed by Morioka et al., there is a problem in that fiber lengths of the two polarization-maintaining single-mode optical fibers must be strictly regulated, and fabrication of the actual device is complicated. In addition, there is a problem with instability of switch operations, due to polarization cross-talk components that are present in the actual polarization-maintaining single-mode optical fibers, as described in Arai Shin-ichi et al., “Polarization Maintaining Fiber”, Furukawa Denko Jiho (Furukawa Electric Review, Japanese version), No. 109, pp. 5-10, January 2002.
As a method for solving these problems, an optical switch as referred to in Japanese Patent Application Laid-Open (JP-A) No. 2006-58508 has been proposed.
That is, JP-A No. 2006-58508 discloses an optical switch in which regulation of fiber lengths of polarization-maintaining single-mode optical fibers that constitute an optical switch is unnecessary, and even if a long strip-form fiber is used as a polarization-maintaining single-mode optical fiber in which the optical Kerr effect is produced, instability of switch operations due to polarization cross-talk components does not arise.
Many different kinds of encoding formats for optical signals to be used in optical communications systems have been proposed and employed. Representative examples thereof are an amplitude modulation system, which represents a binary digital signal by magnitude relationships of peak intensities of an optical signal, and a phase modulation system, which represents a binary digital signal by optical phase differences in an optical carrier wave of an optical signal.
Of the amplitude modulation system and the phase modulation system, it is desirable to suitably select and employ whichever provides maximum satisfaction of required specifications of respective networks. Optical communications networks include modes in which plural networks with different specifications optically communicate to one another. Thus, it is desirable for optical communications networks to mix and employ different optical signals encoded with different modulation formats, such as an amplitude modulation system, a phase modulation system and the like, whichever are suitable for respective locations.
In consideration of these circumstances, it is desirable for an optical modulator that generates encoded optical signals to have general applicability so as to be applicable to either system, of an amplitude modulation system and a phase modulation system.
The all-optical light switches disclosed in the aforementioned Morioka et al. and JP-A No. 2006-58508 utilize optical pulse signals encoded by an amplitude modulation system based on magnitude relationships of peak intensities as control lights. Therefore, these switches may be used as all-optical light intensity modulators that generate amplitude-modulated modulated optical signals.
However, it is difficult to use these all-optical light switches as all-optical optical phase modulators that generate phase-modulated modulated optical signals.
The cross phase modulation effect caused by the optical Kerr effect, which is the principle of operation of these all-optical light switches, can also be used as the principle of operation of a phase modulator, without alteration. Therefore, an all-optical optical phase modulator that utilizes this effect may be provided.
However, in such a case, for compatibility with both an amplitude modulation system and a phase modulation system, separate all-optical light modulators corresponding to the respective systems must be prepared. This leads to increases in size of equipment, rises in costs and increases in power consumption, which are problematic.
If it is possible to realize an all-optical amplitude/phase modulator that is capable, using a single apparatus, of generating optical signals in formats for both an amplitude modulation system and a phase modulation system with a simple means of adjustment, the above problem can be solved. For such a case, it is desirable in practice if this is not associated with significant changes in light losses and the like in accordance with changes in modulation format, or significant changes in signal quality. Furthermore, if characteristics do not vary even with changes in signal light wavelength, environmental temperature and the like and highly stable operation characteristics can be assured, great advantages can be obtained in practice without increases in component numbers, costs and power consumption in relation to stabilization control.
Accordingly, the present invention provides an all-optical optical modulator that is capable of generating optical signals in formats for both an amplitude modulation system and a phase modulation system, with a simple means of adjustment, and of which operations are highly stable. Furthermore, an optical signal generation apparatus that uses this optical modulator is provided.